Avengers needed build-up to establish an interest in their characters. Nobody knew any of their characters besides the one audiences didn't care much for anymore. Justice League has Batman and Superman. Putting them onscreen together will get asses in seats regardless. That's when you add in Wonder Woman, Green Lantern, The Flash, Aquaman, or whoever else and get people interested in them through Black 'n Blue. Come for the World's Finest stay for the spin-offs.

I think a JL movie can very much succeed without solos after all it has Bats and Supes.But there is no doubt in my mind it will have succeeded a whole lot more with solo movies leading up to it.

The appeal of seeing JL movie without solos-Bats and supes
The appeal of seeing JL movie with solos-Bats,supes,flash,GL,WW.

look at both and you can see why the solo movie approach is superior.The plus side is that with the spinoff approach is that its means the spinoff movie are less likely to fail financially(that doesnt mean theyll be good though).

The other problem the spinoff aproach has is establsihing the various mythologies of each hero in the same universe with one go.For example Establishing Wonderwoman mythos and Green lantern in the same universe and having them interact normally.